This invention relates to a method and apparatus for the improved manufacture of cement in wet or dry process long rotary kilns, or in preheater kilns, for processing a mineral material therein to produce cement clinker. More particularly this invention is directed to improvements in the operation and configuration of long rotary cement kilns which have been modified to burn a supplemental fuel in a secondary burning zone of the kiln.
In the widely used commercial process for the manufacture of cement, the steps of drying, calcining, and clinkering cement raw materials are accomplished by passing finely divided raw materials, including calcareous minerals, silica and alumina, through a heated, inclined rotary vessel or kiln. In what is known as conventional long dry or wet process kilns the entire mineral heating process is conducted in a heated rotating kiln cylinder, commonly referred to as a "rotary vessel." The rotary vessel is typically 10 to 15 feet in diameter and 400-700 feet in length and is inclined so that as the vessel is rotated, raw materials fed into the upper end of the kiln cylinder move under the influence of gravity toward the lower "fired" end where the final clinkering process takes place and where the product cement clinker is discharged for cooling and subsequent processing. Kiln gas temperatures in the fired clinkering zone of the kiln range from about 1300.degree. to about 1600.degree. C. Kiln gas temperatures are as low as about 250.degree. to 350.degree. C. at the upper mineral receiving end of so-called wet process kilns. Somewhat higher kiln gas temperatures exist in the upper end of long dry process kilns.
Generally, skilled practitioners consider the cement making process within the rotary kiln to occur in several stages as the raw material flows from the cooler gas exit mineral feed end to the fired/clinker exit lower end of the rotary kiln vessel. As the mineral material moves down the length of the kiln it is subjected to increasing kiln gas temperatures. Thus in the upper portion of the kiln cylinder where the kiln gas temperatures are the lowest, the in-process mineral materials first undergo a drying process and thereafter move down the kiln cylinder until the temperature is raised to calcining temperature. The length of the kiln where the mineral is undergoing a calcining process (releasing carbon dioxide) is designated the calcining zone. The in-process mineral finally moves down the kiln into a zone where gas temperatures are the hottest, the clinkering zone adjacent the fired lower end of the kiln cylinder. The drying zone, the calcining zone, and the clinkering zone typically are not contiguous lengths along the kiln cylinder. The kiln gas stream flows counter to the flow of in-process mineral materials from the clinkering zone, through the intermediate calcining zone and the mineral drying zone and out the upper gas exit end of the kiln into a kiln dust collection system. The flow of kiln gases through the kiln can be controlled to some extent by a draft induction fan positioned in the kiln gas exhaust stream.
Cement kilns have received favorable review from both federal and state environmental regulatory agencies for disposal of both liquid and solid combustible waste materials. Cement kilns provide a combination of high operating temperatures and long residence times, both of which are favorable conditions for complete combustion of organic components of waste and chemical combination of inorganic components with the reactive in-process mineral components. Combustible waste solids provide a source of inexpensive energy for the mineral processing industry.
For many years, regulation compliant use and disposal of waste in operating kilns was limited to combustible liquid or "pumpable" hazardous waste. Liquid waste materials are easily blended with each other and with conventional fuels to provide homogeneous liquids that can be burned in the gaseous phase at the firing end of the kiln with little or no modification of kiln burner configuration. Solid waste derived fuel, however, can occur in multiplicity of forms, from hard crystalline solids to viscous, sticky sludges. They are not easily blended, and they present significant engineering challenges for their safe handling and delivery into rotating kilns.
Several apparatus have been designed to facilitate firing of solid wastes into kilns. See, for example, U.S. Pat. No. 4,850,290, issued Jul. 25, 1989; U.S. Pat. No. 4,930,965, issued Jun. 5, 1990; U.S. Pat. No. 4,974,529, issued Dec. 4, 1990; and U.S. Pat. No. 5,083,516 issued Jan. 7, 1992; incorporated herein by reference. These four patents describe an apparatus and method for delivering containerized waste to both pre-heater type and conventional long wet or dry kilns at the point in the process where the kiln gas temperature is such that volatilized components are consumed with high destruction and removal efficiency. The above-cited patents describe a device for delivering containerized waste through the wall of a kiln cylinder during kiln operation. The apparatus comprises a fuel charging port, preferably with a mechanical closure, formed in the kiln cylinder wall. The fuel charging port is aligned with a drop tube inside the kiln cylinder. The drop tube prevents hot in-process mineral material in the kiln from escaping through the fuel charging port or contacting closure. The device is utilized to deliver containerized waste into a secondary burning zone of the kiln at predetermined time during kiln cylinder rotation.
Other apparatus and methods for delivering solid fuels, especially solid waste derived fuels, are known in the art. See, for example, U.S. Pat. No. 5,078,594, issued Jan. 7, 1992, incorporated herein by reference. The '594 patent discloses a charging apparatus for delivering tires or other combustible waste solids through a port into a rotating kiln. U.S. Pat. No. 5,339,751 owned by the assignees of the present invention discloses a waste derived fuel delivery control method and apparatus capable of charging more than one container or other type of solid waste fuel into the kiln for each revolution of the kiln. In addition, solid waste derived fuel can be fired from an industrial cannon mounted at the lower fired end of the kiln to form a secondary burning zone downstream of the clinkering zone of the kiln. See U.S. Pat. Nos. 5,086,716 and 4,984,983 for examples of industrial cannon fuel delivery systems.
Supplemental fuel added to the kiln may also include natural fossil fuels, such as lump coal, delivered to the rotating kiln. U.S. Pat. No. 5,257,586 discloses one such supplemental fuel delivery apparatus.
Burning a supplemental fuel, either a natural fossil fuel such as lump coal or a waste derived fuel, charged into the kiln reduces fuel consumption of the primary burner at the lower fired end of the kiln. Environmentally sound burning requires complete combustion of the supplemental fuel and optimization of combustion conditions to minimize discharge of carbon monoxide and other products of incomplete combustion into the atmosphere, as well as nitrogen oxides produced in the hot flame of the primary burner.
Responsive to environmental concerns and more rigorous regulating of emission standards, the cement industry has invested in a significant research and development effort to reduce emissions from cement kilns. The present invention provides a method and improved apparatus for improving combustion efficiency and reducing emission of gaseous pollutants during the manufacture of cement in conventional long rotary kilns. Air is injected through a port in the wall of a rotary cement kiln having a secondary burning zone established by burning solid fuel in contact with the in process mineral at a point downstream, relative to kiln gas flow of the clinkering zone in the kiln. The air injection apparatus is designed to enhance mixing of the injected air with the kiln gas stream and is characterized by high linear flow rates and/or multi-directional air injection ports. The air injection ports can be located downstream of a secondary burning zone to provide reduction in concentration of NO.sub.x, CO, SO.sub.2 and hydrocarbons in the kiln gas stream. Heretofore reduction of NO.sub.x levels in combustion gas effluents has typically been at the cost of higher levels of one or more of the other gaseous contaminants.
One object of the present invention is to promote cross sectional mixing of injected air with the kiln gas stream near a secondary burning zone.
Another object of the present invention is to reduce emissions of carbon monoxide, sulfur dioxide, hydrocarbons, and nitrogen oxides from a kiln which has been modified to burn a supplemental fuel.
Yet another object of the present invention is to increase the oxygen level adjacent the secondary burning zone to support complete combustion of a supplemental fuel charged into the secondary burning zone without disrupting the thermal profile of the burner flame in the primary burning zone at the lower fired end of the kiln.
Still object of the present invention is to increase the amount of supplemental waste derived fuel which may be charged into the kiln so that the burner intensity near the discharge end can be reduced, thereby reducing the amount of fuel consumption by the burner.
Still another object of the present invention is to supply tertiary air to the rotary vessel without the use of complicated sealing arrangements to prevent escape of the tertiary air.
According to one aspect of the present invention, a method is provided for reducing the concentration of nitrogen oxides, as well as a carbon monoxide and other products of incomplete combustion, in the effluent gas emissions during operation of a conventional long rotary cement kiln. The kiln includes a rotary vessel having an upper kiln gas exit end with an adjacent mineral drying zone, an opposite fired clinker exit end with an adjacent clinkering zone and an intermediate mineral calcining zone. During operation, a kiln gas stream flows from said clinkering zone through the calcining zone and out the kiln gas exit end of the rotary vessel. In-process mineral flows from the drying zone through the clinkering zone. The method includes the steps of delivering combustible material into the rotary vessel to burn it in contact with the in-process mineral downstream relative to kiln gas flow of the clinkering zone to create a reducing zone extending downstream from the burning combustible material, injecting air through at least one port in a wall of the rotary vessel and directing said injected air into the kiln gas stream at a point upstream relative to kiln gas flow of the drying zone at a linear flow rate of at least 100 feet per second.
In the illustrated method, the injected air is directed through an air injection tube in air flow communication with the port in the wall of the rotary vessel. The injected air is directed into the kiln gas stream at multiple angles so that at least a portion of the injected air is directed toward the in-process mineral continuously during kiln operation. Illustratively, the injected air is directed radially from a point proximal to the rotational axis of the rotary vessel toward the wall of the rotary vessel.
In one illustrated method, the combustible material is delivered through a port in the wall of the rotary vessel into the calcining zone of the kiln. The combustible material may be a conventional fossil fuel, a combustible waste, or a containerized combustible waste.
The distance between the point of contact of the combustible material with the in-process mineral and a point of injection of air into the kiln gas stream is illustratively at least X, where X=(0.5 second).multidot.R, and where R is the average linear flow rate of the kiln gas stream in the rotary vessel. The air must be injected into the kiln gas stream at a point at which the temperature of the kiln gas stream is at least the minimum temperature required to burn (oxidize) carbon monoxide in the kiln gas stream. In one embodiment air is injected in accordance with this invention at a point where the kiln gas temperature is at least 1600.degree. F. preferably at a point downstream relative to kiln gas flow from the calcining zone. Preferably, the distance between an upstream end relative to kiln gas flow of the reducing zone and the point of injection of air into the kiln gas stream is about (0.5 second).multidot.R to about (3 seconds).multidot.R, where R is the average linear flow rate of the kiln gas stream in the rotary vessel.
According to another aspect of the present invention, a method is provided for reducing the concentration of nitrogen oxides in effluent gas emissions during operation of a conventional long rotary cement kiln. The kiln includes a rotary vessel having an upper kiln gas exit end with an adjacent mineral drying zone, and opposite fired clinker exit end with an adjacent clinkering zone and an intermediate mineral calcining zone. During operation of the kiln, a kiln gas stream flows from said clinkering zone through the calcining zone and out the kiln gas exit end of the rotary vessel. In-process mineral flows from the drying zone through the clinkering zone. The method includes the steps of delivering combustible material into the rotary vessel to burn it in contact with in-process mineral at a point downstream relative to kiln gas flow of the clinkering zone to create a reducing zone extending downstream from the burning combustible material, injecting air through at least one port in the wall of the rotary vessel and directing said injected air into the kiln gas stream upstream relative to kiln gas flow of the drying zone at multiple angles so that at least a portion of the injected air is directed toward the in-process mineral continuously during kiln operation. The method optionally includes the step of promoting mixing of the kiln gas stream as it passes through the reducing zone, for example, installing a choker ring in the reducing zone of a rotary vessel to provide an axial length of reduced cross-sectional area, or by installing baffles or other means to mix upper and lower laminar portions of the kiln gas stream as it moves through the reducing zone.
According to yet another aspect of the present invention, an apparatus is provided for the improved manufacture of cement in a wet or dry process long rotary kiln for processing a mineral material therein to produce cement clinker. The kiln includes a rotary vessel having a fired lower end and an adjacent clinkering zone, an upper kiln gas exit end and an adjacent mineral drying zone, an intermediate calcining zone along its length, and a kiln gas stream flowing from the fired lower end to the upper gas exit end. The kiln is modified to burn a supplemental fuel in a secondary burning zone of the kiln located downstream relative to kiln gas flow from the clinkering zone. The apparatus includes a port formed in the rotary vessel upstream relative to kiln gas flow of the drying zone, an air injection tube in air flow communication with the port, and a fan in air flow communication with the air entry port to inject air into the rotary vessel through the port and the air injection tube. The apparatus also includes means coupled to the air injection tube for directing the injected air into the kiln gas stream at multiple angles relative to a longitudinal axis of rotation of the rotary vessel.
In the illustrated embodiment, the air injection tube is configured to prevent in-process mineral material from passing through the port, and the fan is mounted on the rotary vessel. In one illustrated embodiment, the directing means includes a deflector mounted on an air discharge end of the air injection tube inside the rotary vessel to disperse the air entering the rotary vessel through the air injection tube at multiple angles relative to the longitudinal axis of rotation of the rotary vessel. In another illustrated embodiment, the rotary vessel includes an array of air entry ports angularly spaced on a surface of the rotary vessel and a circumferential air supply duct extending around the rotary vessel in air flow communication with the fan and the array of air entry ports. In yet another illustrated embodiment, the directing means includes at least two air injection nozzles mounted on an air discharge end of the air injection tube inside the rotary vessel. The nozzles are aligned at different angles relative to the longitudinal axis of rotation of the rotary vessel. In an illustrated embodiment, the fan has a capacity to inject the air into the kiln gas stream at a linear flow rate of at least 100 feet per second. Preferably, the fan has a capacity to inject the air into the kiln gas stream at a linear flow rate of about 200 feet per second to about 500 feet per second.
According to still another aspect of the present invention, an apparatus is provided for the improved manufacture of cement in a wet or dry process long rotary kiln for processing a mineral material therein to produce cement clinker. The kiln includes a rotary vessel having a fired lower end and an adjacent clinkering zone, an upper kiln gas exit end and an adjacent mineral drying zone, an intermediate calcining zone along its length, and a kiln gas stream flowing from the fired lower end to the upper gas exit end. The kiln is modified to burn a supplemental fuel in a secondary burning zone of the kiln located downstream relative to kiln gas flow from the clinkering zone. The apparatus includes a port formed in the rotary vessel at a point upstream relative to kiln gas flow of the drying zone, an air injection tube in air flow communication with the port, and a fan in air flow communication with the air entry port to inject air into the rotary vessel through the port and the air injection tube. The apparatus also includes at least one air injection nozzle coupled to the air injection tube. The at least one nozzle injecting air into the kiln gas stream at a linear flow rate of at least 100 feet per second.
In the illustrated embodiment, a plurality of air injection nozzles are coupled to an air discharge end of the air injection tube inside the rotary vessel. The nozzles are aligned at multiple angles relative to a longitudinal axis of rotation of the rotary vessel. In another illustrated embodiment, a rotary valve coupled to the air discharge end of the air injection tube. The rotary valve is configured to direct air flow from at least one of the plurality of nozzles continuously toward the in-process mineral during rotation of the rotary vessel. The rotary valve has a stationary valve body including a plurality of air flow passageways. Each air flow passageway is aligned in air flow communication with a selected one of the plurality of nozzles. The rotary valve also includes a gravity actuated movable plate configured to block air flow into upwardly directed passageways of the valve body during rotation of the rotary vessel.
According to another aspect of the present invention, an apparatus is provided for the improved manufacture of cement in a wet or dry process long rotary kiln for processing a mineral material therein to produce cement clinker. The kiln includes a rotary vessel having a fired lower end and an adjacent clinkering zone, an upper kiln gas exit end and an adjacent mineral drying zone, an intermediate calcining zone along its length, and a kiln gas stream flowing from the fired lower end to the upper gas exit end. The kiln is modified to burn a supplemental fuel in a secondary burning zone of the kiln located downstream relative to kiln gas flow from the clinkering zone. The apparatus includes a port formed in the rotary vessel at a point located a predetermined distance downstream relative to kiln gas flow from an upstream end of the secondary burning zone. The predetermined distance is between about (0.5 second).multidot.R and about (3 seconds).multidot.R, where R is the average linear flow rate of the kiln gas stream in the rotary vessel. The apparatus also includes an air injection tube in air flow communication with the port, and a fan in air flow communication with the air entry port to inject air into the rotary vessel through the port and the air injection tube.
According to another aspect of the present invention, an apparatus is provided for the improved manufacture of cement in a wet or dry process long rotary kiln for processing a mineral material therein to produce cement clinker. The kiln includes a rotary vessel having a fired lower end and an adjacent clinkering zone, an upper kiln gas exit end and an adjacent mineral drying zone, an intermediate calcining zone along its length, and a kiln gas stream flowing from the fired lower end to the upper gas exit end. The kiln is modified to burn a supplemental fuel in a secondary burning zone of the kiln located downstream relative to kiln gas flow from the clinkering zone. The apparatus includes a port formed in the rotary vessel upstream relative to kiln gas flow of the drying zone, a fan in air flow communication with the air entry port to inject air into the rotary vessel through the port, and an air injection tube in air flow communication with the port. The air injection tube is configured to impart rotational momentum to the injected air about a longitudinal axis of the rotary vessel. In the illustrated embodiment, the air injection tube is configured to inject air underneath the in-process mineral during rotation of the rotary vessel.
Additional objects, features and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of the preferred embodiments exemplifying the best mode of carrying out the invention as presently perceived.